Age-related variations in δ13C of ecosystem respiration across a coniferous forest chronosequence in the Pacific Northwest

We tested the hypothesis that forest age influences the carbon isotope ratio (delta(13)C) of carbon reservoirs and CO2 at local and regional levels. Carbon isotope ratios of ecosystem respiration (delta(13)C(R)), soil respiration (delta(13)C(R-soil)), bulk needle tissue (delta(13)C(P)) and soil organic carbon (delta(13)C(SOC)) were measured in > 450-, 40- and 20-year-old temperate, mixed coniferous forests in southern Washington, USA. Values of delta(13)C(R), delta(13)C(R-soil), delta(13)C(P) and delta(13)C(SOC) showed consistent enrichment with increasing stand age. Between the youngest and oldest forests there was a similar to1parts per thousand enrichment in delta(13)C(P) (at similar canopy levels), delta(13)C(SOC) (throughout the soil column), delta(13)C(R-soil) (during the wet season) and delta(13)C(R) (during the dry season). Mean values of delta(13)C(R) were -25.9, -26.5 and -27.0parts per thousand for the 450-, 40- and 20-year-old forests, respectively. Both delta(13)C(R-soil) and the difference between delta(13)C(R) and delta(13)C(R-soil) were more 130 enriched in older forests than in young forest: delta(13)C(R)-delta(13)C(R-soil) = 2.3, 1.1 and 0.5parts per thousand for the 450-, 40- and 20-year-old forests, respectively. Values of delta(13)C(P) were proportionally more depleted relative to delta(13)C(R): delta(13)C(R)-delta(13)C(P) = 0.5, 2.2 and 2.5parts per thousand for the 450-, 40- and 20-year-old forests, respectively. Values of delta(13)C(P) were most C-13-enriched at the top of the canopy and in the oldest forest regardless of season (overall values were -26.9, -28.7 and -29.4parts per thousand for the 450-, 40- and 20-year-old forests, respectively). Values of delta(13)C(SOC) from shallow soil depths were similar to delta(13)C(P) values of upper- and mid-canopy needles. All delta(13)C data are consistent with the hypothesis that a decrease in stomatal conductance associated with decreased hydraulic conductance leads to increased CO2 diffusional limitations in older coniferous trees. The strong associations between delta(13)C(P) in needles with delta(13)C(R) and delta(13)C(R-soil) at the forest level suggest that 130 observations scale between leaf and ecosystem levels.